The present invention is directed to a capacitor having a metallurgical bond formed by welding an aluminum terminal to a zinc coating, wherein the zinc coating has been thermally spayed on the common edge of a multi-layer plate in a capacitor element. The invention is particularly useful in solid state capacitors.
Capacitors may be produced by winding together two metalized plastic films, to form a cylindrical shape. By way of example, the metalized film may be a polypropylene film substrate having a zinc, aluminum or zinc-aluminum alloy coating, created by vapor deposition of the metal. The two films are offset or staggered slightly so that the edges do not overlap, thereby creating a common edge at either end of the cylinder. The ends of the cylinder are then surface coated with a layer of zinc or zinc alloy, typically by a thermal spraying technique, such as arc spaying. The coating formed on the end of the cylinder is sometimes referred to as an “end spray.”
Terminals are attached to the coating on the common edge, at each end of the capacitor. Both terminals can be placed on the same side of the cylinder, for example, by running the bottom terminal through an insulated tube down the center of the cylinder. The terminals or leads are typically a metal strip or tab. Terminals comprising a single metal or an alloy of two or more metals have been employed. Alternatively, the terminal may be plated with a second metal or comprise a laminate structure of two or more metals, in an effort to enhance the compatibility of the metals sought to be joined.
One prior art method of attaching a terminal to a zinc end spray is by soldering. The bond generated by soldering gets its strength from the filler metal and its affinity for the surfaces to which it is bonded. There are two important conditions for the bond to occur. First, the filler metal (solder) must be melted, and second, the molten solder must wet the surface or surfaces that are to be joined. The wettability of a surface is influenced by its cleanliness, as well as the attraction of the solder to the metals to be joined.
Fluxes are often used to remove barriers, such as primarily oxides, between the filler metal and the bonding surface during soldering. High temperatures during soldering tend to promote fluidity and penetration of the solder, and aggressive fluxes tend to be more efficient at oxide removal. Both of these conditions, however, also promote damage of the metalized substrate (e.g. thermoplastic film) comprising the capacitor element, by melting and contamination of the film and its metallization, thereby reducing the performance of the capacitor. These considerations have led to the development of low melting solders and fluxes with benign residues. For example, copper terminals have been soldered to a zinc coating on a capacitor using near-eutectic lead-tin solder. The melting point of eutectic lead-tin solder is 183° C. This temperature is high enough to melt polypropylene film, but if the volume of solder is small, the time of heating is kept short and the end spray coating is sufficiently thick, then thermal damage to the film underneath the end spray can be minimized.
Regulations mandating the removal of lead from solder compositions have been enacted, such as The Restriction of the Use of Certain Hazardous Substances in Electrical and Electronic Equipment Regulations 2006 (“RoHS Regulations”) promulgated by the European Union. Unfortunately, employing materials in compliance with the regulations greatly raises the melting temperature of the solder and increases the likelihood of damage to capacitors containing a thermoplastic film substrate. These new solders also require more aggressive fluxes which can have a negative effect on reliability and life.
Various approaches have been used in the industry to achieve a good terminal attachment while mitigating film damage. To improve wetting, common edge coatings (end sprays) containing tin, such as a tin/zinc alloys have been used. Alternately, tin-rich coatings have been flame or arc sprayed on top of the initial zinc end spray. Both of these techniques present a tin-rich surface that is very wettable—provided the surfaces are protected against oxidation or hydration. To mitigate melting of the underlying film, a thicker coating on the common edge of the capacitor element has been used. Obviously this adds cost and weight, and is of limited value. Another drawback to employing tin or a tin alloy in the end spray is the lack of solubility between tin and certain other metals, which inhibits a strong metallurgical bond from forming.
The copper terminals that are used on high current film capacitors, such as power DC film capacitors rated hundreds of microfarads and tens of amps of ripple current, are typically 0.25 to 1.0 mm in thickness, in order to carry the load current without overheating. Various attempts have been made to weld a copper terminal to the zinc coated common edge of a capacitor. For example, a tin-plated copper terminal has been welded to a zinc coating using parallel electrodes, in a process referred to as “stitch welding.” The energy required to weld the components can be minimized, in order to avoid damaging the thermoplastic film, by thinning the copper terminal to a thickness of about 0.25 mm at the welding site.
For good bonding to occur there should be some solubility between the dissimilar metals comprising the weld, for example copper-tin-zinc. Tin was thought to play a significant role in the terminal welding process. Tin forms an eutectic liquid with zinc at 198° C., and upon cooling the system forms a two phase bond between tin and zinc. The relationship between tin and copper is more complex. A eutectic between copper and tin occurs at 227° C., and upon cooling, the bond is composed of two phases. The first phase is a tin solid solution and the second phase is the inter-metallic, phase Cu6Sn5, which is also known as the eta prime (η′) phase at room temperature.
Initially, an assumption was made that the primary reaction at the interface was between tin and zinc, which formed a zinc/tin alloy at the surface. Energy Dispersive X-ray analysis (“EDX”), however, shows that the bonding preferentially occurs between the reaction of copper and zinc. The tin has greater solid solubility with copper and was found to be associated with the copper epsilon phase in the bond. The zinc-tin alloy is in the form of two distinct phases and is believed to be relatively unstable. Another drawback of using tin-plated copper terminals is the tendency of tin to oxidize, making it more difficult to work with.
The bond between the terminal and the coating on the end spray (common edge of the capacitor element) must meet minimum “pull strength” requirements, that is, the terminal must be able to withstand being separated from the end spray coating when the terminal is physically pulled. The bond between the terminal and coating must remain strong over time and resist oxidation and corrosion.
In addition to the electrical connection between the terminal and the zinc coating on the common edge of the capacitor element, the opposite end of the terminal is typically connected to a header. The header may be made of brass. Thus, a further requirement of many terminals used in capacitors is that they can be welded to a brass fixture, within the capacitor header.